Joshua 8:3

So Joshua arose, and all the people of war, to go up against
Ai
As the Lord had commanded him:

and Joshua chose out thirty thousand mighty men of valour;
out of all the men of war; these were a select company, picked men, not the whole army, as some have thought, for he was ordered to take all the people of war, as he did:

and sent them away by night;
from the main army, that they might pass the city and get behind it undiscovered, for they were sent for an ambush; and of these some were to take the city, and be left in it to burn it, and some to smite the men of Ai, as Abarbinel notes.